1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method for driving the same, and more particularly, to an LCD device that can remove residual images from a screen by rapidly discharging a low gate voltage applied to a gate line and a method for driving the same.
2. Discussion of the Related Art
Generally, LCD devices display images by controlling light transmittance through liquid crystals by controlling the electric field in the liquid crystal cells. The LCD device includes an LCD panel that is provided with liquid crystal cells arranged in a matrix configuration and a driving circuit that drives the LCD panel. The LCD panel includes a thin film transistor (TFT) that is formed adjacent to each crossing of gate and data lines and a pixel electrode that is connected to the TFT. The TFT includes a gate electrode that is connected to any one of the gate lines by each horizontal pixel-line and a source electrode that is connected to any one of the data lines by each vertical pixel-line. The TFT supplies a data signal from the data line to the pixel in response to a gate driving pulse of the gate line.
The pixel has a pixel electrode that is connected to a drain electrode of the TFT, a common electrode that faces the pixel electrode, and a liquid crystal layer disposed therebetween. The pixel drives the liquid crystal in response to the data signal supplied to the pixel electrode, thereby controlling the light transmittance. Hereinafter, a related art LCD device will be explained with reference to the accompanying drawings.
FIG. 1 is a circuit diagram illustrating one pixel of an LCD device according to the related art. As shown in FIG. 1, each pixel of the LCD device is defined by a gate line (GL) and a data line (DL) that are formed perpendicularly. Each pixel includes a TFT and a pixel electrode. In particular, the TFT is formed adjacent to each crossing of the gate and data lines (GL, DL). Also, the TFT includes a gate terminal connected to the gate line (GL), a source terminal connected to the data line (DL), and a drain terminal connected to the pixel electrode 160.
The LCD device includes first and second glass substrates being bonded to each other and a liquid crystal layer formed between the two glass substrates. FIG. 1 shows one pixel formed on the first substrate of a TFT array substrate. Although not shown, the second substrate includes an R/G/B color filter layer that represents various colors and a common electrode 150 that displays images. The pixel electrode 160 is provided facing the common electrode 150. A liquid crystal layer is disposed between the pixel electrode 160 and the common electrode 150. Based on the intensity of the electric field between the pixel electrode 160 and the common electrode 150, the light transmittance through the liquid crystal can be controlled.
A storage capacitor (Clc) is formed by the common electrode 150, the pixel electrode 160, and the liquid crystal layer. The common electrode 150 functions as a first electrode of the storage capacitor (Clc). The pixel electrode 160 functions as a second electrode of the storage capacitor (Clc). The liquid crystal layer functions as a dielectric of the storage capacitor (Clc). Each pixel electrode 150 partially overlaps with the gate line (GL) to drive the adjacent pixel. An insulator is provided between the pixel electrode 160 and the gate line (GL). The overlapping portion between the gate line (GL) and the pixel electrode 160 functions as an auxiliary storage capacitor (Cst). A predetermined portion of the pixel electrode 160 functions as a first electrode of the auxiliary storage capacitor (Cst). A predetermined portion of the gate line (GL) functions as a second electrode of the auxiliary storage capacitor. The insulator functions as a dielectric of the auxiliary storage capacitor. Each pixel electrode 160 overlaps with the gate line (GL) of the adjacent pixel, which is referred to as a previous gate structure.
An operation of the above-mentioned pixel will be explained as follows. First, the TFT is turned-on when a high gate voltage is applied to the gate line (GL). Once the TFT is turned-on, a data voltage of the data line (DL) is supplied to the pixel electrode 160. Accordingly, the storage capacitor (Clc) is charged with a predetermined voltage, i.e., a voltage corresponding to the difference between the data voltage applied to the pixel electrode 160 and the voltage applied to the common electrode 150.
On the other hand, the TFT is turned-off when a low gate voltage is applied to the gate line (GL). When the TFT is turned-off, the storage capacitor (Clc) is not being charged. However, the voltage of the storage capacitor (Clc) maintains the previous voltage. Accordingly, the gray scale of the frame is maintained. However, within a predetermined time after the TFT is turned-off, a voltage drop occurs between both terminals of the storage capacitor due to leakage of current. Since the auxiliary storage capacitor (Cst) is also charged, this predetermined time is determined by the natural exponential decay rate of the auxiliary storage capacitor (Cst). In other words, the voltage drop is not rapid.
FIG. 2 is a graph of illustrating the voltage-current properties of a TFT according to the related art. As shown in FIG. 2, when the high gate voltage (VGH) is applied to the gate terminal of the TFT, the TFT is in the on-current (Ion) state. In this state, the voltage of the source terminal is applied to the storage capacitor (Clc). When the low gate voltage (VGL) is applied to the gate terminal of the TFT, the TFT is in the off-current (Ioff) state. In this state, the electric charges of the storage capacitor (Clc), which were charged when the high gate voltage (VGH) was applied to the gate terminal, are not discharged to the outside.
However, the related art LCD device has the following disadvantages when the source voltage is blocked after the driving the related art LCD device. Just before blocking the source voltage of the LCD device, a low gate voltage (VGL) is applied to most of gate lines (GL). In other words, a high gate voltage (VGH) is applied to only one gate line that is selected and a low gate voltage (VGL) is supplied to other gate lines except for the one selected. Therefore, the low gate voltage (Voff) is applied to the gate terminals provided in most of the TFTs. For LCD devices according to the related art, the auxiliary storage capacitor (Cst) is charged with the low gate voltage (VGL). Accordingly, until discharging the low gate voltage (VGL) from the auxiliary storage capacitor (Cst) to reach a ground voltage, a low gate voltage (VGL) is applied to the gate terminal of the TFT. Accordingly, it is impossible to make the prompt response of removing the displayed image. In other words, since the auxiliary storage capacitor (Cst) is maintained with the low gate voltage (VGL), it is difficult to promptly discharge the auxiliary storage capacitor. For this reason, the related art LCD device using the previous gate structure has the residual image after the source voltage is blocked.